This invention relates to a positioning device for positioning an article in alignment with a predetermined point on the basis of the peripheral shape of the article. More particularly, the invention is concerned with a positioning device for positioning a photomask or reticle having a pattern to be transferred onto a semiconductor wafer, in alignment with the center of rotation of a rotatable mask holder.
In the field of manufacture of semiconductor devices such as integrated circuits, the photomask or reticle (which hereinafter will be referred to simply as "mask") having a pattern to be transferred onto the semiconductor wafer is positioned in its place on the basis of the peripheral shape thereof. This is also the case with the semiconductor wafer. For example, in a projection type exposure apparatus for exposing the wafer to the pattern of the mask, each of the mask and the wafer is pre-positioned on the basis of its peripheral shape before it is moved to the pattern transfer station. By this pre-positioning, the center of the mask or the wafer is aligned with the center of rotation of a holder for holding the mask or the wafer.
The mechanism for such pre-positioning usually includes a plurality of positioning pins which are adapted to engage with the peripheral edge of the mask or the wafer. If these positioning pins are fixed relative to the holder, it is possible that the edge portion of the mask or the wafer may impinge against these positioning pins when the mask or wafer is moved to the holder. This is undesirable.
It has been proposed to make these positioning pins movable relative to the holder, such as disclosed in IBM Technical Disclosure Bulletin Vol. 17, No. 12, p. 3577 published May 1975. The proposed positioning device includes six positioning pins which are retractable radially outwardly with respect to a holder or chuck. With this arrangement, the problem as described above can be obviated. However, since these six positioning pins are driven by a common cam, there still remain the following problems. First, the proposed positioning device could not easily meet the change in size and/or shape of the wafer or the mask. This is a significant disadvantage because, in recent years, masks of rectangular shape and masks of circular shape are used in the same exposure apparatus and because there is a tendency in production to increase the diameter of each of the wafer and the mask.
Second, a large cam is required to drive the positioning pins, which leads to bulkiness of the positioning device. This is particularly undesirable where the positioning device is to be incorporated into the exposure apparatus.